Boreholes are commonly drilled into the ground to recover hydrocarbons, such as oil and gas, from subterranean formations. Such boreholes are usually drilled with a drill bit at the end of a drill string. The drill string can be formed on-site by consecutively adding any number of tubular members (sometimes also referred to as segments of drill pipe). The lower end of the drill string commonly includes a bottomhole assembly, having any number of drill string tools, with the drill bit attached to the bottom end. The drill bit is rotated, such as by rotating the drill string or by independently rotating the drill bit using a mud motor, to shear or disintegrate material of the rock formation to drill the wellbore.
Some tools and devices included in a drill string require remote activation and deactivation during drilling operations. Examples of such tools and devices include reamers, stabilizers, and force application members used for steering the drill bit. The harsh downhole environment, however, routinely poses a challenge for designers of electro-mechanical control systems, to achieve a desired level of performance and reliability.
Various methods have been devised for remotely operating tools using controlled fluid pressure. The use of controlled fluid pressure in the drill string often allows a limited number of activation/deactivation cycles, after which the control system is to be reset. Some reamer activation apparatuses, for example, use a ball-drop mechanism that permits a single activation cycle, after which a reset of the control system is required. In many conventional systems, the drilling fluid (i.e. “mud”) cycled down the drill string and back up a borehole annulus can be used as the control fluid. In such systems, the drilling mud can perform multiple separate functions, with corresponding drilling fluid pressure levels. In addition to pressurization of the drilling mud to circulate it through the drill string and the annulus, drilling mud pressure and flow can, for example, be varied to control mud motor speed and/or torque. Because of such multiple, distinct reasons for variations in drilling mud pressure during drilling operations, using drilling mud to control a tool or device actuation mechanism can cause inadvertent tool activation resulting from misinterpretation of unrelated mud pressure fluctuations as actuating mechanism control signals.